(1) Field of the Invention
This invention relates to a radiation counter which can be used in a high temperature environment and, particularly, a radiation counter technique which can realize radiation counters with extremely high effectiveness that satisfy the specification required at an operating temperature exceeding 450.degree. C., afford a large pulse amplitude of the output pulse current and have a short collection time of electron charge (hereinafter, referred to as "a short electron-collection-time").
(2) Description of the Prior Art
In the radiation counter of the prior art, such as the nuclear fission counter, that was used nearly at the ordinary temperature or at the medium temperature range not exceeding about 450.degree. C., fast electron-collection-gases containing the argon gas with some amount of the diatomic gas or the polyatomic gas are generally used as the counter gas to obtain good operating performances, such as the large output pulse current and the short electron-collection-time, of the counter. The argon gas containing the carbonic acid gas of 2% to 5% in volume and the nitrogen of 1% to 2%, for examples, are well known as such fast electron-collection-gases. Sometimes, the fast electron-collection-gas is simply called as the fast counter gas. The reason why such good operating performances of the counter are obtained when the fast counter gas is used is that the electron drift velocity in these fast counter gases is relatively very high compared with that in the innert gas such as the pure argon gas and, naturally, in the counter operating performances, the electron-collection-time becomes short and the induced pulse current of the counter electrode or the output current of the counter is large.
Meanwhile, in the advanced nuclear power reactors, such as the fast breeding reactors (hereinafter, referred to as FBRs) and the high temperature gas cooled reactors (hereinafter, referred to as HTGRs), etc. that are recently being developed, the core temperature of those reactors becomes to be designed very high to enhance the heat utilization efficiency of the reactors and the reactor development programs require, for the reactor control purpose, to develop the heat-resisting nuclear radiation counters, especially neutron counters which can be used in such high temperature environment in those reactors. In order to cope with this situation, many efforts to increase heat-resistivity of the radiation counters are continued in many countries in the world. As some results of these efforts, high temperature radiation counters wherein the operating temperature exceeds 450.degree. C. are developed in England (e.g. P 7A type fission counter) and in France (e.g. CFU-12 type fission counter).
Pure argon gas is, however, used as the counter gas in the high temperature heat-resisting radiation counters of the prior art such as mentioned above and, therefore, the amplitude of the output pulse currect of those counters is about one-third as small as and the electron-collection-time is about three times as long as those of the same kind of counters that are designed to be used at the ordinary and/or medium temperature ranges up to 450.degree. C. This is the serious defect in the operating performance of the high-temperature heat-resisting radiation counters of the prior art as described later.
In the prior art, it is widely believed that the high temperature radiation counter filled with the fast electron-collection-gas cannot be used in the high temperature environment exceeding 450.degree. C. as described in page 21 of the literature "Neutron Detectors for Reactor Control" written by D. J. Mobbs, J. Inst. Nucl. Eng., Vol. 18, No. 1, pp 16 to 25, 1977. The fast electron-collection-gas is composed of inert gases as the chief ingredient and some amount of the diatomic gas or the polyatomic gas, such as the nitrogen and the carbon acid gas. However, in the conventional radiation counters, those mixed gases present phenomena to react with the electrodes and the structural metal in the high radiation and high temperature environment exceeding 450.degree. C. More particularly, the nitrogen is easily absorbed into the electrodes etc. through the nitrogenizing process and the carbon acid gas is dissolved and also absorbed into the metal structure in a short time through the carbonizing and sintering process in such rigorous and stern environment. By this reason, as the contents of the diatomic gas and the polyatomic gas in the counter gas are rapidly decreased in such rigorous and stern environment, the character of the gas in the counter, such as the electron drift velocity in the gas etc. is substantially changed and no stable operation an naturally be maintained in the ordinary radiation counters of the prior art.
The problems to be caused by the fact that the collection time of the radiation counter becomes long and the output pulse current small are described hereinafter. The short electron-collection-time characteristics of the counter is, of course, important to reduce the pulse pile-up in the counter itself. However, in case that the amplitude of the output pulse current of the counter becomes small, it further affects remarkably to the overall performances of the counting system in which the counter is used.
If the amplitude of the output pulse currect of the counter is decreased by a factor 1/n, the pulse-shaping time constants of the pulse amplifier must be, in general, increased by a factor n.sup.2 under the same condition of the signal-to-noise ratio, because the frequency bandwidth of the amplifier is inversely proportional to the shaping time constant, while the amplitude of the output noise is proportional to a square-root of the bandwidth of the amplifier.
Consequently, since the pulse-width broadens by a factor n.sup.2 and the pulse pile-up probability also increases by a factor n.sup.2, the maximum counting rate that can be measured decreases to 1/n.sup.2 and, furthermore, the permissible maximum fluence rate of the background radiation, such as gamma-rays and alpha-rays in neutron measurement, also decreases to 1/n.sup.2. Thus, as the performance of the counting system is largely affected by the amplitude of the output pulse current of the radiation counter used, various efforts have been made in various countries in the world to develop high performance and heat-resisting nuclear radiation counters satisfying both the large pulse current output-characteristics and high temperature stability. However no one succeeded in this development.